In designing a vehicle, the constructor carries out a number of tests aiming to improve the behaviour of the vehicle. These tests comprise so-called objective tests that make it possible to quantitatively qualify the behaviour of the vehicle and so-called subjective tests that can be used to qualitatively qualify the behaviour of the vehicle. The subjective tests are performed by testers who drive the vehicle and qualify its behaviour on the basis of their impression.
The objective tests notably comprise braking, road holding and safety tests. The subjective tests notably comprise headings relating to the straight-line behaviour, the general behaviour and active safety. The straight-line behaviour includes manoeuvres performed for low lateral accelerations of the vehicle, for example so-called direction, linearity, driving approval, centring, and other such manoeuvres.
The purpose of the heading relating to the straight-line behaviour is notably to qualify the response of the vehicle as a function of a stress, generally of low angle amplitude on the steering wheel exerted by the tester, in other words, for example, how the torque on the steering wheel or the yaw of the vehicle vary qualitatively and quantitatively in response to the angle variation on the steering wheel. Thus, for example, a vehicle exhibiting a linear response exhibits a behaviour that is generally judged to be satisfactory by the tester whereas a vehicle for which the response is non-existent for a small angle of rotation of the steering wheel exhibits a behaviour that is generally judged to be unacceptable to the tester.
When the tester judges the straight-line behaviour of the vehicle to be unacceptable, the vehicle is modified then retested by the tester in order for the latter to check whether the vehicle exhibits a satisfactory behaviour following the modifications made. Since the cause of the unacceptable behaviour is not known to the tester, the requests for modifications to be made derive from the experience of said tester. They relate non-exhaustively to the tyres, the wheels, the steering system, the geometry and the elements of the suspension device. The steps listed above are repeated until the vehicle exhibits a straight-line behaviour that is satisfactory to the tester.